I’d like to build a Microsoft health module to pull info from my Microsoft account for display on my mirror. I’ve started researching how to do this, but confess to being a bit over my head with it at the moment. Can someone point me in the right direction?

There you go then. Look at the Cloud API, specifically the Javascript example. That’s what you need to start writing your own module. You could also post a request in the Modules/Request forum section and see if someone might want to help you write one.